There is conventionally known an acceleration sensor that includes a rectangular parallelepiped weight portion having a movable electrode; a pair of beam portions for rotatably supporting the weight portion substantially at a center in the longitudinal direction of the weight portion; and a pair of fixed electrodes arranged in a spaced-apart opposing relationship with respect to one side and the other side of the surface of the weight portion demarcated by a straight line interconnecting the beam portions (see, e.g., Japanese Patent Application Publication No. 2008-544243).
One prior art example of the acceleration sensor will now be described with reference to FIGS. 4 and 5. In the following description, upper and lower sides in FIG. 4 will be defined as an up-down direction, the direction parallel to the width direction of a sensor chip 1 as an x-direction, the direction parallel to the longitudinal direction of the sensor chip 1 as a y-direction and the direction orthogonal to the x-direction and the y-direction as a z-direction. As shown in FIGS. 4 and 5, the acceleration sensor of the prior art example includes the sensor chip 1 having a rectangular plate-like outward appearance; an upper fixed plate 2a fixed to the upper surface of the sensor chip 1; and a lower fixed plate 2b fixed to the lower surface of the sensor chip 1.
The sensor chip 1 includes a frame portion 3 having two rims 3a and 3b formed into a rectangular shape when seen in the up-down direction and arranged side by side along the longitudinal direction; rectangular parallelepiped weight portions 4 and 5 arranged adjacent to each other inside the rims 3a and 3b in a spaced-apart relationship with respect to the inner circumferential surfaces of the rims 3a and 3b; two pairs of beam portions 6a and 6b, and 7a and 7b for respectively interconnecting the inner circumferential surfaces of the rims 3a and 3b and the side surfaces of the weight portions 4 and 5 to rotatably support the weight portions 4 and 5 with respect to the frame portion 3; and movable electrodes 4a and 5a formed on the upper surfaces of the weight portions 4 and 5.
Each of the weight portions 4 and 5 includes a recess section 41 or 51 opened in one surface (the lower surface) thereof and a solid section 40 or 50 one-piece formed with the recess section 41 or 51. The recess section 41 or 51 is formed to have a rectangular plan-view shape when seen in the direction normal to the open surface (in the up-down direction). Reinforcing walls 42 and 52 for bisecting the inside of the recess sections 41 and 51 are respectively one-piece formed with the weight portions 4 and 5.
One pair of the beam portions 6a and 6b interconnects the rim 3a and the substantially central sections in the x-direction of the side surfaces of the weight portion 4 facing the rim 3a. Similarly, another pair of the beam portions 7a and 7b interconnects the rim 3b and the substantially central sections in the x-direction of the side surfaces of the weight portion 5 facing the rim 3b. Accordingly, the straight line interconnecting the beam portions 6a and 6b and the straight line interconnecting the beam portions 7a and 7b respectively become rotation axes about which the weight portions 4 and 5 rotate.
The sensor chip 1 is formed by processing a SOI (Silicon-On-Insulator) substrate by a semiconductor fine processing technology. The sections including the upper surfaces of the weight portions 4 and 5 respectively become the movable electrodes 4a and 5a. Protrusions 43a and 43b, and 53a and 53b for preventing the weight portions 4 and 5 from directly colliding with the upper fixed plate 2a and the lower fixed plate 2b are respectively provided to protrude from the upper and lower surfaces of the weight portions 4 and 5.
The upper fixed plate 2a is made of an insulating material, e.g., glass, and is provided at the side of the movable electrodes 4a and 5a, i.e., above the sensor chip 1 in the illustrated example. On the lower surface of the upper fixed plate 2a, first and second fixed electrodes 20a and 20b are arranged side by side in the x-direction in such positions as to face the weight portion 4 (the movable electrode 4a) of the sensor chip 1 along the up-down direction. First and second fixed electrodes 21a and 21b are arranged side by side in the x-direction in such positions as to face the weight portion 5 (the movable electrode 5a) of the sensor chip 1 along the up-down direction. At one x-direction end side of the upper fixed plate 2a, five through-holes 22a through 22e are formed to extend through the upper fixed plate 2a in the y-direction to penetrate through the upper fixed plate 2a. On the lower surface of the upper fixed plate 2a, there is formed a plurality of conductive patterns (not shown) electrically connected to the respective fixed electrodes 20a, 20b, 21a and 21b. 
On the other hand, four electrode units 8a, 8b, 9a and 9b spaced apart from the frame portion 3 are arranged side by side at one x-direction end side of the sensor chip 1. Detection electrodes 80a, 80b, 90a and 90b made of metal films are formed substantially at the centers of the upper surfaces of the four electrode portions 8a, 8b, 9a and 9b, respectively. Pressure contact electrodes 81a, 81b, 91a and 91b made of metal films are formed on the upper surfaces of the end sections of the four electrode units 8a, 8b, 9a and 9b facing the rims 3a and 3b. The detection electrode 80a (80b) and the pressure contact electrode 81a (81b) are connected to each other.
An earth electrode 10 is formed on the upper surface of the frame portion 3 between the electrode units 8b and 9a. The earth electrode 10 is electrically connected to the movable electrode 4a through the beam portions 6a and 6b and to the movable electrode 5a through the beam portions 7a and 7b. If the upper fixed plate 2a is bonded to the upper surface of the sensor chip 1, the conductive patterns formed on the lower surface of the upper fixed plate 2a are connected, by pressure contact, to the pressure contact electrodes 81a, 81b, 91a and 91b. Thus the respective detection electrodes 80a, 80b, 90a and 90b are electrically connected to the fixed electrodes 20a, 20b, 21a and 21b and are exposed to the outside through the through-holes 22a through 22d of the upper fixed plate 2a. The earth electrode 10 is also exposed to the outside through the through-hole 22e. 
Similar to the upper fixed plate 2a, the lower fixed plate 2b is made of an insulating material such as glass or the like. The lower fixed plate 2b is provided at the opposite side of the sensor chip 1 from the upper fixed plate 2a, i.e., below the sensor chip 1. Adherence-preventing films 23a and 23b are respectively formed on the upper surface of the lower fixed plate 2b in positions corresponding to the weight portions 4 and 5 of the sensor chip 1 along the up-down direction. The adherence-preventing films 23a and 23b are made of the same material as the fixed electrodes 20a, 20b, 21a and 21b, e.g., aluminum-based alloy. The adherence-preventing films 23a and 23b serve to prevent the lower surfaces of the rotated weight portions 4 and 5 from adhering to the lower fixed plate 2b. 
In the conventional example stated above, the rim 3a, the weight portion 4, the beam portions 6a and 6b, the movable electrode 4a, the first and second fixed electrodes 20a and 20b and the detection electrodes 80a and 80b make up one sensor unit. The rim 3b, the weight portion 5, the beam portions 7a and 7b, the movable electrode 5a, the first and second fixed electrodes 21a and 21b and the detection electrodes 90a and 90b make up another sensor unit. Two sensor units are one-piece formed with each other in a state that the orientations of the weight portions 4 and 5 (the arrangements of the solid sections 40 and 50 and the recess sections 41 and 51) are 180 degrees inverted on the same plane.
Description will now be made on the detection operation of the prior art example. First, it is assumed that acceleration is applied to the weight portion 4 in the x-direction. If acceleration is applied in the x-direction, the weight portion 4 rotates about the rotation axis thereof, thereby changing the distances between the movable electrode 4a and the first and second fixed electrodes 20a and 20b. As a result, capacitances C1 and C2 between the movable electrode 4a and the respective fixed electrodes 20a and 20b are also changed. In the regard, the capacitances C1 and C2 at the time of application of acceleration in the x-direction can be represented by the following equations:C1=C0−ΔC  Eq. 1andC2=C0+ΔC  Eq. 2,
where C0 denotes the capacitance between the movable electrode 4a and the respective fixed electrodes 20a and 20b when acceleration is not applied in the x-direction and ΔC denotes the capacitance change generated by the application of acceleration.
Similarly, the capacitances C3 and C4 between the movable electrode 5a and the respective fixed electrodes 21a and 21b at the time of application of acceleration to the weight portion 5 in the x-direction can be represented by the following equations:C3=C0−ΔC  Eq. 3andC4=C0+ΔC  Eq. 4.
In this connection, the values of the capacitances C1 through C4 can be detected by arithmetically processing the voltage signals extracted from the detection electrodes 80a, 80b, 90a and 90b. Then, the sum (±4ΔC) of a differential value CA (=C1−C2) between the capacitances C1 and C2 acquired from one of the sensor units and a differential value CB (=C3−C4) between the capacitances C3 and C4 acquired from the other sensor unit is calculated. Based on the sum of the differential values CA and CB, it is possible to calculate the direction and magnitude of the acceleration applied in the x-direction.
Next, it is assumed that acceleration is applied to the weight portion 4 in the z-direction. If acceleration is applied in the z-direction, the weight portion 4 rotates about the rotation axis thereof, thereby changing the distances between the movable electrode 4a and the first and second fixed electrodes 20a and 20b. As a result, capacitances C1 and C2 between the movable electrode 4a and the respective fixed electrodes 20a and 20b are also changed. In the regard, the capacitances C1′ and C2′ at the time of application of acceleration in the z-direction can be represented by the following equations:C1′=C0′−ΔC′  Eq. 5andC2′=C0′+ΔC′  Eq. 6,
where C0′ denotes the capacitance between the movable electrode 4a and the respective fixed electrodes 20a and 20b when acceleration is not applied in the z-direction and ΔC′ denotes the capacitance change generated by the application of acceleration.
Similarly, the capacitances C3′ and C4′ between the movable electrode 5a and the respective fixed electrodes 21a and 21b at the time of application of acceleration to the weight portion 5 in the z-direction can be represented by the following equations:C3=C0−ΔC  Eq. 7andC4=C0+ΔC  Eq. 8.
Then, the difference (±4ΔC) of a differential value CA′ (=C1′−C2′) between the capacitances C1′ and C2′ acquired from one of the sensor units and a differential value CB′ (=C3′−C4′) between the capacitances C3′ and C4′ acquired from the other sensor unit is calculated. Based on the difference of the differential values CA′ and CB′, it is possible to calculate the direction and magnitude of the acceleration applied in the z-direction. The arithmetic processing for finding the direction and magnitude of the acceleration applied in the x-direction and the z-direction using the sum of the differential values CA and CB and the difference of the differential values CA′ and CB′ is well-known in the art and, therefore, will not described in detail herein.
In the meantime, in the conventional acceleration sensor stated above, the respective electrode units 8a, 8b, 9a and 9b spaced apart from the weight portions 4 and 5 and the frame portion 3 and electrically insulated from the sensor chip 1 are connected to the fixed electrodes 20a, 20b, 21a and 21b by way of the pressure contact electrodes 81a, 81b, 91a and 91b. The capacitances C1 through C4 are detected through the detection electrodes 80a, 80b, 81a and 81b provided in the respective electrode units 8a, 8b, 9a and 9b. However, the spaces required to provide the respective electrode units 8a, 8b, 9a and 9b occupy about 30% to 40% of the area of the sensor chip 1. This poses a problem in that it becomes difficult to reduce the size of the acceleration sensor. Even if the size of the respective weight portions 4 and 5 is reduced, it is still necessary to provide the spaces for installation of the respective electrode units 8a, 8b, 9a and 9b. Thus difficulties are involved in reducing the size of the acceleration sensor.